In gas turbine engines air is compressed and fuel burned in the high pressure air. This gas is then expanded through a gas turbine. Such a gas turbine has a plurality of alternate rotor stages for withdrawing energy and stator stages for redirecting the flow.
It is desirable that the entire gas flow pass through the stator stage to achieve the proper orientation upon entering the succeeding rotor stage. Gas bypassing the stator vanes represents a power and efficiency loss and therefore is to be minimized.
It is conventional to establish an inner shroud connecting the inner edge of all vanes. A seal between the shroud and the rotating rotor must be provided. Often circumferential knife edges on the rotor in close proximity to an abradable strip on the stator are used to restrict flow between the shroud and the rotor.
Segmented shrouds and segmented abradable strips have leakage therebetween and present sealing problems in attempting to minimize this leakage. Full annular shrouds are therefore desired for this purpose. A full annular inner shroud in conjunction with vanes secured thereto and the outer shroud of the turbine stage creates a relatively rigid structure. Temperature differentials existing throughout these components produce strains which must be absorbed. Such strains may be minimized by modifying expansion by use of appropriate amounts of cooling air to the selected components. In conventional construction such growth change for the purpose of limiting the strain of these components can lead to additional leakage past the inner shroud.
Over the operating range of the gas turbine varying steady state and transient temperature distributions occur in the various components. Minimum leakage is obtained with freedom to choose material with the desired coefficient of expansion and predicted expansion. Improved design results may be obtained if this problem can be divorced from the strain problem of the vanes and the bounding shrouds. Segmented rings can have relative movement between the segments and therefore the expansion is less predictable than it would be on a full annular ring.